The present invention relates to a light-emitting device and a process for production thereof and, more particularly, to a light-emitting device having an emission layer held between electrodes and a process for production thereof.
An organic electroluminescence (referred to as EL hereinafter) device, which is a spontaneous light-emitting device (referred to as light-emitting device hereinafter), has an organic film including an organic emission layer between an anode and a cathode. FIG. 4 is a sectional view showing the structure of one example of such light-emitting devices. The light-emitting device shown in this figure has a lower electrode 102 as an anode of metallic material formed on a substrate 101 and an organic layer 106 including an organic hole injection layer 103, an organic hole transporting layer 104, and an organic emission layer 105 or the like, which are sequentially laid one over the other on the lower electrode 102. And, on this organic layer 106 is formed an upper electrode 107 as a cathode which is a thin film of metallic material transparent to light and is further formed a transparent conductive film 108 to reduce the resistance of the upper electrode 107 as a cathode.
The light-emitting device of such structure is a display element of so-called xe2x80x9ctop emitting typexe2x80x9d, which works in such a way that the light generated by the organic emission layer 105 is reflected by the lower electrode 102 of metallic material and is allowed to emanate through the upper electrode 107 placed opposite to the substrate 101.
Moreover, production of the light-emitting device of such structure starts with forming a layer of metallic material on the substrate 101 by any process adequately selected from various processes such as sputtering, resistance heating vapor deposition, and electron beam vapor deposition, and forming the lower electrode 102 by patterning the layer of metallic material. Subsequent steps are sequential deposition of materials for the organic layers 103 to 105, the upper electrode 107, and the transparent conductive film 108 through a deposited mask, and the deposition is followed by patterning to form the organic layer 106, the upper electrode 107, and the transparent conductive layer 108.
However, the display device and the process for production thereof as mentioned above have the following problem. Namely, all the layers of metallic material tend to have polycrystalline structure no matter whether they are formed by sputtering, resistance heat vapor deposition, or electron beam vapor deposition. As the result, the lower electrode 102 formed by patterning the layer of metallic material has projections on its surface due to marked surface roughening, as shown in FIG. 5 (an enlarged sectional view), although not so serious as the transparent anode formed from ITO (indium tin oxide).
Consequently, the organic layer 106 formed on the lower electrode 102 becomes locally thin at spots corresponding to projections, with the result that the distance d across the organic layer 106 between the lower electrode 102 and the upper electrode 107 becomes locally short and the electric field concentrates at such spots to cause leakage current.
This leakage current does not help the light-emitting device to emit light but decreases the efficiency of the light-emitting device. Moreover, extremely concentrated leakage current results in a short circuit between the lower electrode 102 and the upper electrode 107, thereby disabling the light-emitting device from light emission. This is the cause for non-emitting spots so-called dark spots in the organic EL display.
A possible way to prevent leakage current is to form the lower electrode 102xe2x80x2 from an electrically conductive material, such as chromium oxide, which gives a flat film surface, as shown in FIG. 6. However, the electrically conductive material in oxide form has a low reflectivity and readily transmits light, so that the lower electrode 102xe2x80x2 made of the electrically conductive material permits the light h reaching it from the emission layer 105 as one of the organic layers 106 to be absorbed toward the substrate 101. This results in a decrease in the amount of light h emanating from the upper electrode 107, which in turn leads to a decease in emission efficiency.
It is an object of the present invention to provide a light-emitting device of top emission type and a process for production thereof, the light-emitting device keeping a stable emitting efficiency without leakage current.
The present invention to achieve the above-mentioned object covers a light-emitting device composed of a lower electrode which is laid on a substrate, an organic layer including at least an emission layer which is laid on the lower electrode, and an upper electrode transparent to light which is laid on the organic layer, characterized in that the lower electrode is of laminate structure consisting of a metallic material layer and a layer of buffering thin film which is formed on the metallic material layer. The buffering thin film is made of any oxide of the metallic material constituting the metallic material layer, the oxide having a higher conductivity than the organic layer, or made of an oxide of chromium.
Also, according to the present invention, the light-emitting device is produced by a process including steps of forming a lower electrode on a substrate, forming on the lower electrode an organic layer including an emission layer, and forming above the substrate an upper electrode transparent to light such that the emission layer is held between the lower electrode and the upper electrode, characterized in that the step of forming the lower electrode includes additional steps of forming a buffering thin film on a metal film formed on the substrate and patterning the metal film and buffering thin film, thereby forming the lower electrode of laminate structure consisting of the metallic material layer and the buffering thin film. The buffering thin film is made of any oxide of the metallic material constituting the metal layer, the oxide having a higher conductivity than the organic layer, or made of an oxide of chromium.
The above-mentioned light-emitting device and production process are characterized in that the surface layer of the lower electrode is a buffering thin film which is made of any oxide of the metal constituting the underlying metallic material layer, the oxide having a higher conductivity than the organic layer, or made of an oxide of chromium. In general, a metal film has a rough surface due to polycrystalline structure, whereas a film of an oxide of the metallic material constituting the metal film has a less rough surface. In addition, a buffering thin film made of an oxide of chromium has a less rough surface than the metal film independently of the metal film underneath. Moreover, the buffering thin film functions as the lower electrode if it is made of any oxide of the metal constituting the metallic material layer, the oxide having a higher conductivity than the organic layer, or made of a chromium oxide which has the highest conductivity among oxides. Thus, the foregoing structure ensures a uniform space between the lower electrode whose metallic material layer has a mildly rough surface owing to the buffering thin film layer constituting the surface layer and the upper electrode transparent to light which is laid on the lower electrode with an organic layer interposed between them.